We propose research that will investigate molecular properties of the cell surface receptor for immunoglobulin E (IgE) and the changes in those properties which occur to trigger mast cell degranulation. The work will involve developing procedures for labeling the IgE-receptor complex at specific sites with a variety of radioactive and fluorescent reagents. The effect of these specific modifications on the ability of the IgE-receptor complex to mediate the triggering signal will be evaluated in an assay for functional capability which we will develop. This assay will employ membrane-cell fusion procedures to reincorporate isolated receptors into whole cells where they may be tested for activity. This approach should allow us to determine which of the specific modifications affect sites in the receptor that are critical for its function. We will further utilize the specifc modifications that are characterized to introduce fluorescent probes at well-defined sites on the IgE-receptor complex, so that resonance energy transfer distance measurements may be carried out to determine spatial relationships between these sites. By these means, we hope to construct a three-dimensional picture of the structure of this membrane-bound protein complex.